This invention relates generally to electric motors, and more particularly, to methods for die casting squirrel cage rotors for electric induction motors.
At least one type of rotor used in electric induction motors includes a rotor core and a plurality of die cast metal rotor bars integrally extending within slots in the rotor core between metal end rings. When electrical windings of a stator are energized, a voltage is induced in the rotor bars, which creates a magnetic field and causes the rotor core to rotate. A shaft is attached to the rotor core and coupled to drive components for driving transmission thereof. The bars are often skewed relative to the end rings to facilitate rotation of the rotor in a predetermined direction and to generate a pre-selected starting torque of the motor. Because of the shape of the metal bars and end rings, these types of rotors are often referred to as xe2x80x9csquirrel cagexe2x80x9d rotors.
The performance of induction motors including squirrel cage rotors is closely related to the integrity of the bars and the end rings and the connections between the bars and the end rings. For example, if molten metal freezes prematurely during die casting of the squirrel cage, an air gap between one or more of the bars and the end rings may be created. Thus, an xe2x80x9copen barxe2x80x9d or an open circuit is created by the gap and the electrical performance of the rotor is impaired. As another example, a xe2x80x9ccold shutxe2x80x9d bar may result from thin layers of solidified metal, oxides and debris at a leading edge of a flowing molten metal front. When two such metal fronts meet, such as when a bar fills with molten metal simultaneously from both end rings during die casting, the two fronts may incompletely fuse, which increases electrical resistance of a bar and degrades electrical performance of the rotor. Both open bars and cold shut bars are problematic with conventional die casting techniques.
Accordingly, it would be desirable to provide a die cast method for forming squirrel cage induction rotors that reduces occurrences of open bars and decreases electrical resistance of the squirrel cage by inhibiting formation of cold shut bars.
In an exemplary embodiment of the invention, a gating system for vertical die casting of a squirrel cage rotor includes an upper end ring cavity, a lower end ring cavity, and a plurality of passages in fluid communication with the end ring cavities for forming rotor bars. A rotor core including a plurality of rotor bar slots is positioned within the gating system so that the rotor slots extend between the upper and lower cavities and form the passages in fluid communication with the upper and lower end ring cavities.
At least one gate is located a radial distance from the passages so that molten metal may be injected into the gating system lower end ring cavity. Because the gate is located a radial distance from, or separated from, the passages, the lower end ring cavity fills with molten metal before the molten metal fills the passages. As molten metal is continued to be injected, each of the plurality of passages, i.e., the rotor core slots, are filled with molten metal at approximately the same time and at approximately the same rate until the passages are full and the upper end ring cavity fills with molten metal.
Open bars are therefore avoided because molten metal reaches the upper end ring cavity through each of the passages at approximately the same time. Therefore, freezing of the molten metal in any one of the passages is unlikely. In addition, formation of cold shut bars is avoided because molten metal fronts flow through each of the bars in only one direction from the lower end ring cavity toward the upper end ring cavity. Rather than meeting within a reduced cross sectional area of the rotor bar passages, molten metal fronts meet in the relative large cross sectional area of the upper end ring cavity where fusing of the metal fronts is much less of a concern. Thus, electrical resistance of the bars is decreased due to the absence of cold shut bars, which enhances electrical performance of the rotor.